Jim Phillips writes:

>Mark Landin wrote:
>
>>Now I *have* heard some chatter on comp.sys.hp.hpux by Bill Hassell (I
>>think) about what happens when you mix 90-m and 120-m tapes. It seems
>>the tape mechanism or electronics need time to "adapt" to a media type
>>when you've been using a different media type.
>
>Hmmmmm.  "Adapting" electro-mechanical products?  HP has come a long
>way.  What's next?  Tape cloning? :-)
>
>I'm sorry, but this sounds like so much equine fertilizer to me.  If
>the tape drive can handle a 120M tape, then why not a 90M tape?

Because the media itself is different. A 120M tape isn't just a 90M tape
with an extra 30M glued onto the end; it's made from a different material.

From Appendix C of the HP7980 family reference manual:

   Hewlett-Packard supports the use of 1-mil (3600-ft reel) tapes on
   the HP 7979A/7980A/7980XC tape drives only under certain conditions...

   Two characteristics of 1-mil tape must be taken into account
   before this tape is used... thin tapes conform to the read/write
   heads differently and therefore wear the heads differently
   than 1.5-mil tapes.

   ...With regard to different head wear patterns, the HP7979A/
   HP7980A/HP7980XC heads are affected by the use of 1-mil tape
   in the same way as heads on any other tape drive; the critical
   read/write area of the head wears at an increased rate and
   forms a different profile from that made by 1.5-mil tape.

   When a 1.5-mil tape is mounted on a drive in which the
   read/write area of the head has been worn by frequent use of
   1-mil tape, the thicker 1.5-mil tape cannot conform to the
   wear profile caused by the 1-mil tape and will pass over the
   read/write area of the head at a greater distance. This
   increased tape-to-head distance causes signal loss. The effect
   of signal loss can be an increase in read and write errors.
   This effect is true for all industry-standard half-inch
   tape drives.

   Because of the incompatibility of the head wear profiles,
   Hewlett-Packard can support the use of 1-mil tapes on the
   HP7979A, HP7980A, and HP7980XC drives only if the following
   guidelines are used:

   * if a significant portion (more than 1 tape in 10) of the
     tapes used on the drive are 1-mil tapes, we recommend
     that a drive be dedicated to the use of the thinner tapes.

   * if less than 1 tape in 10 used on the drive is a 1-mil tape,
     AND at least 10 1.5-mil tapes are mounted between the
     mountings of the 1-mil tapes, the two tape types can be
     used on the same drive. [Emphasis in original.]

This is not necessarily the case for DATs, but it does indicate a
mechanism by which electromechanical components adapt.

-- Bruce


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